Introducing their study, Smith et al. (2013) state that "fine root production of forest ecosystems comprises around one third of global annual net primary productivity in terrestrial ecosystems, highlighting the importance of roots in the global carbon cycle (Jackson et al., 1997)," and they say that "in a number of studies using single tree species, fine root biomass has been shown to be strongly increased by elevated CO2." However, they additionally state that "natural forests are often intimate mixtures of a number of co-occurring species," and they felt driven to learn what happens in such a multi-species situation. More specifically, "to investigate the interaction between tree mixture and CO2," as Smith et al. describe it, "Alnus glutinosa [black alder], Betula pendula [European white birch] and Fagus sylvatica [European beech] were planted in areas of single species and a three-species polyculture in a free-air CO2 enrichment study (BangorFACE)," where "the trees were exposed to ambient or elevated CO2 (580 ppm) for four years," during which time "fine and coarse root biomass, together with fine root turnover and fine root morphological characteristics were measured."

In describing their findings, the five researchers report that at the end of the final growing season, "the overall effect of the elevated CO2 treatment was a 31% increase in fine root biomass," but they say that "in the polyculture fine root biomass was enhanced by 68% under elevated CO2." Smith et al. thus conclude their work by stating that their data suggest that "existing biogeochemical cycling models parameterized with data from species grown in monoculture may be underestimating the belowground response to global change." Indeed, the CO2-induced enhancement of tree root growth out in the real world of nature may actually be much larger than previously believed.